Generally, in a high frequency oscillation type proximity switch, as shown in FIG. 8 illustrating a related art proximity switch, a parallel resonant circuit 1 is formed by a detection coil L and a capacitor C. An oscillation circuit 2 is connected to this parallel resonant circuit to supply a high frequency resonance signal thereto, and this oscillation signal is detected by a detector circuit 3. When a metallic body 4 comes to a certain proximity of the detection coil L, the loss of the detection coil L increases and its oscillation condition changes thereby reducing the amplitude of the oscillation output signal. Therefore, the output of the detector circuit 3 is supplied to a comparator circuit 5 which discriminates the output and supplies a detection signal to an external circuit via an output circuit 6. Thus, approach of an object can be detected from the drop in the amplitude of the oscillation circuit 2.
According to such a high frequency oscillation type proximity switch, the electromagnetic loss of the detection coil increases by the approach of a metallic object, in particular by the approach of a magnetic object such as iron and steel members, but is relatively insensitive to nonmagnetic objects, and the presence of nonmagnetic members such as aluminum objects may not be detected. In recent years, as a result of the effort to reduce the weight of various pieces of equipment, more and more aluminum members are being used as work pieces to be handled in manufacturing plants, and there is a growing need to be able to detect such objects.
Normally, an approach of an iron object to a detection coil causes an increase in the loss of the detection coil, but an approach of an aluminum object to a detection coil causes an increase in the susceptance of the detection coil. On the other hand, the increase in the loss of the coil due to the approach of an aluminum object is so small and the resulting change in the oscillation amplitude is so small that the detection sensitivity has been considered to be too low for most practical purposes.